Apache HTTP Server

Server Pool Management

We found that many people were using values for "MaxServers" either too high or too low, and were hanging themselves on it. The model we adopted is still based on long-lived minimal-forking processes, but instead of specifying one number of persistent processes, the web-master specifies a maximum and minimum number of processes to be "spare" - every couple of seconds the parent checks the actual number of spare servers and adjusts accordingly. This should keep the number of servers concurrently running relatively low while still ensuring minimal forking.

We renamed the current StartServers to MinSpareServers, created separate StartServers parameter which means what it says, and renamed MaxServers to MaxSpareServers (though the old name still works, for NCSA 1.4 back-compatibility). The old names were generally regarded as too confusing.

The defaults for each variable are:

MinSpareServers         5
MaxSpareServers         10
StartServers            5
There is an absolute maximum number of simultaneous children defined by a compile-time limit which defaults to 256 and a "MaxClients" directive which specifies the number of simultaneous children that will be allowed. MaxClients can be adjusted up to the compile-time limit (HARD_SERVER_LIMIT, defined in httpd.h). If you need more than 256 simultaneous children, you need to modify both HARD_SERVER_LIMIT and MaxClients.

In versions before 1.2, HARD_SERVER_LIMIT defaulted to 150.

We do not recommend changing either of these values unless:

  1. You know you have the server resources to handle more
  2. You use the machine for other purposes and must limit the amount of memory Apache uses

Apache HTTP Server